There are many type of backlights used for liquid crystal displays (LCDs). Generally, for full color backlights, the light used to illuminate the backlight has red, green, and blue components. Fluorescent lamps are most commonly used as the light source. With the development of high power LEDs, such LEDs have been replacing the fluorescent lamps in some applications. A combination of red, green, and blue LEDs may be used, or “white light” LEDs may be used. A white light LED uses a blue or UV LED coated with a wavelength-converting phosphor so that the resulting light appears white.
A typical backlight for a small or medium size LCD uses a solid, transparent light guide formed of a polymer. The light source, either a fluorescent bulb or LEDs, is optically coupled to one side edge of the rectangular light guide. The light guide may be in the shape of a wedge or have facets or other types of reflectors that uniformly leak light out of the face of the light guide onto the liquid crystal layers. The red, green, and blue pixel locations of the liquid crystal layers are controlled by electrical signals to effectively act as light shutters for the RGB pixels to create a color image on the LCD screen.
FIG. 1 illustrates a typical LCD using one or more LEDs in the backlight. The light source is an LED module 10. The module 10 comprises an LED die 12 attached to a submount 14, housed in a reflective cavity 16. The cavity has a window 18 that is optically coupled to a side edge of a solid light guide 20. The LED 12 may emit blue light, and a phosphor layer (e.g., YAG:Ce) over the LED leaks some of the blue light and emits yellow-green light, so that the combined light is white. Alternatively, there may be multiple color LEDs to supply red, green, and blue light components to the light guide 20.
Metal pads on the LED die 12 are bonded to corresponding pads on the submount 14. The LED die 12 may be a flip-chip, or the bonds may be by wires. The submount 14 has terminals 22 that connect to a printed circuit board (PCB) 24. The PCB 24 has a conductor pattern connecting the terminals 22 to an LED driver 26.
The LED die 12 and submount 14 may alternatively be located in a cavity formed in the solid light guide 20.
A diffuser 28 and brightness enhancing film (BEF) 30 are located above the light guide 20 to make the light more uniform and brighter within a narrow viewing angle.
Liquid crystal layers 32 are located on the BEF 30 film. An LCD driver 34 controls the electric fields at each RGB pixel location to display an image on the LCD.
A bottom reflective film 36 reflects all light in the light guide upwards toward the liquid crystal layers 32. The light guide 20 may have a wedge shape, be roughened on a surface, have facets, or have other deformities to leak out the light through the top surface.
For a very thin and small LCD, such as suitable for cell phones, personal digital assistants (PDAs), digital cameras, and other small devices, the liquid crystal layers may be as thin as 0.75 mm, the diffuser 28 and BEF 30 may have a combined thickness of 0.25 mm, and the light guide 20 and reflector 36 may have a combined thickness of 0.6 mm, for a total thickness of about 1.6 mm. These thicknesses are believed to be the thinnest currently available.
The total height of the LED module 10 may be made as thin as the light guide/diffuser. The PCB 24 adds significant thickness to the LCD since it provides mechanical support to the LED module 10 and the electrical terminals. The PCB may also serve to remove heat from the submount 14, depending on the power level of the LED. The PCB may be as thick as 1.5 mm.
In very thin devices, it is desirable to further reduce the thickness of LCDs.